The present invention relates to methods and compositions for analyzing nucleic acids, and in particular, methods and compositions for detection and characterization of nucleic acid sequences and sequence changes.
The detection and characterization of specific nucleic acid sequences and sequence changes have been utilized to detect the presence of viral or bacterial nucleic acid sequences indicative of an infection, the presence of variants or alleles of mammalian genes associated with disease and cancers, and the identification of the source of nucleic acids found in forensic samples, as well as in paternity determinations. As nucleic acid sequence data for genes from humans and pathogenic organisms accumulates, the demand for fast, cost-effective, and easy-to-use tests for as yet unknown, as well as known, mutations within specific sequences is rapidly increasing.
A handful of methods have been devised to scan nucleic acid segments for mutations. One option is to determine the entire gene sequence of each test sample (e.g., a clinical sample suspected of containing bacterial strain). For sequences under approximately 600 nucleotides, this may be accomplished using amplified material (e.g., PCR reaction products). This avoids the time and expense associated with cloning the segment of interests However, specialized equipment and highly trained personnel are required for DNA sequencing, and the method is too labor-intense and expensive to be practical and effective in the clinical setting.
In view of the difficulties associated with sequencing, a given segment of nucleic acid may be characterized on several other levels. At the lowest resolution, the size of the molecule can be determined by electrophoresis by comparison to a known standard run on the same gel. A more detailed picture of the molecule may be achieved by cleavage with combinations of restriction enzymes prior to electrophoresis, to allow construction of an ordered map. The presence of specific sequences within the fragment can be detected by hybridization of a labeled probe, or the precise nucleotide sequence can be determined by partial chemical degradation or by primer extension in the presence of chain-terminating nucleotide analogs.
For detection of single-base differences between like sequences (e.g., the wild type and a mutant form of a gene), the requirements of the analysis are often at the highest level of resolution. For cases in which the position of the nucleotide in question is known in advance, several methods have been developed for examining single base changes without direct sequencing. For example, if a mutation of interest happens to fall within a restriction recognition sequence, a change in the pattern of digestion can be used as a diagnostic tool (e.g., restriction fragment length polymorphism [RFLP] analysis). In this way, single point mutations can be detected by the creation or destruction of RFLPs.
Single-base mutations have also been identified by cleavage of RNA-RNA or RNA-DNA heteroduplexes using RNaseA (Myers et al., Science 230:1242 [1985] and Winter et al., Proc. Natl. Acad. Sci. USA 82:7575 [1985]). Mutations are detected and localized by the presence and size of the RNA fragments generated by cleavage at the mismatches. Single nucleotide mismatches in DNA heteroduplexes are also recognized and cleaved by some chemicals, providing an alternative strategy to detect single base substitutions, generically named the xe2x80x9cMismatch Chemical Cleavagexe2x80x9d (MCC) (Gogos et al., Nucl. Acids Res., 18:6807-6817 [1990]). However, this method requires the use of osmium tetroxide and piperidine, two highly noxious chemicals which are not suited for use in a clinical laboratory. In addition, all of the mismatch cleavage methods lack sensitivity to some mismatch pairs, and all are prone to background cleavage at sites removed from the mismatch.
RFLP analysis suffers from low sensitivity and requires a large amount of sample. When RFLP analysis is used for the detection of point mutations, it is, by its nature, limited to the detection of only those single base changes which fall within a restriction sequence of a known restriction endonuclease. Moreover, the majority of the available enzymes have 4 to 6 base-pair recognition sequences, and cleave too frequently for many large-scale DNA manipulations (Eckstein and Lilley (eds.), Nucleic Acids and Molecular Biology, vol. 2, Springer-Verlag, Heidelberg [1988]). Thus, it is applicable only in a small fraction of cases, as most mutations do not fall within such sites.
A handful of rare-cutting restriction enzymes with 8 base-pair specificities have been isolated and these are widely used in genetic mapping, but these enzymes are few in number, are limited to the recognition of G+C-rich sequences, and cleave at sites that tend to be highly clustered (Barlow and Lehrach, Trends Genet., 3:167 [1987]). Recently, endonucleases encoded by group I introns have been discovered that might have greater than 12 base-pair specificity (Perlman and Butow, Science 246:1106 [1989]), but again, these are few in number.
If the change is not in a restriction enzyme recognition sequence, then allele-specific oligonucleotides (ASOs), can be designed to hybridize in proximity to the unknown nucleotide, such that a primer extension or ligation event can be used as the indicator of a match or a mis-match. Hybridization with radioactively labeled allelic specific oligonucleotides (ASO) also has been applied to the detection of specific point mutations (Conner, Proc. Natl. Acad. Sci., 80:278 [1983]). The method is based on the differences in the melting temperature of short DNA fragments differing by a single nucleotide (Wallace et al., Nucl. Acids Res. 6:3543 [1979]). Similarly, hybridization with large arrays of short oligonucleotides was proposed as a method for DNA sequencing (Bains and Smith, J. Theor. Biol. 135:303 [1988]) (Drmanac et al., Genomics 4:114 [1989]). To perform either method it is necessary to work under conditions in which the formation of mismatched duplexes is eliminated or reduced while perfect duplexes still remains stable. Such conditions are termed xe2x80x9chigh stringencyxe2x80x9d conditions. The stringency of hybridization conditions can be altered in a number of ways known in the art In general, changes in conditions that enhance the formation of nucleic acid duplexes, such as increases in the concentration of salt, or reduction in the temperature of the solution, are considered to reduce the stringency of the hybridization conditions. Conversely, reduction of salt and elevation of temperature are considered to increase the stringency of the conditions. Because it is easy to change and control, variation of the temperature is commonly used to control the stringency of nucleic acid hybridization reactions.
Discrimination of hybridization based solely on the presence of a mismatch imposes a limit on probe length because effect of a single mismatch on the stability of a duplex is smaller for longer duplexes. For oligonucleotides designed to detect mutation in genomes of high complexity, such as human DNA, it has been shown that the optimal length for hybridization is between 16 and 22 nucleotides, and the temperature window within which the hybridization stringency will allow single base discrimination can be as large as 10xc2x0 C. (Wallace [1979], supra). Usually, however, it is much narrower, and for some mismatches, such as G-T, it may be as small as 1 to 2xc2x0 C. These windows may be even smaller if any other reaction conditions, such as temperature, pH, concentration of salt and the presence of destabilizing agents (e.g., urea, formamide, dimethylsulfoxide) alter the stringency. Thus, for successful detection of mutations using such high stringency hybridization methods, a tight control of all parameters affecting duplex stability is critical.
In addition to the degree of homology between the oligonucleotide probe and the target nucleic acid, efficiency of hybridization also depends on the secondary structure of the target molecule. Indeed, if the region of the target molecule that is complementary to the probe is involved in the formation of intramolecular structures with other regions of the target, this will reduce the binding efficiency of the probe. Interference with hybridization by such secondary structure is another reason why high stringency conditions are so important for sequence analysis by hybridization. High stringency conditions reduce the probability of secondary structures formation (Gamper et al., J. Mol. Biol. 197:349 [1987]). Another way to of reducing the probability of secondary structure formation is to decrease the length of target molecules, so that fewer intrastrand interactions can occur. This can be done by a number of methods, including enzymatic, chemical or thermal cleavage or degradation. Currently, it is standard practice to perform such a step in commonly used methods of sequence analysis by hybridization to fragment the target nucleic acid into short oligonucleotides (Fodor et al., Nature 364:555 [1993]).
Two other methods of mutation detection rely on detecting changes in electrophoretic mobility in response to minor sequence changes. One of these methods, termed xe2x80x9cDenaturing Gradient Gel Electrophoresisxe2x80x9d (DGGE) is based on the observation that slightly different sequences will display different patterns of local melting when electrophoretically resolved on a gradient gel. In this manner, variants can be distinguished, as differences in the melting properties of homoduplexes versus heteroduplexes differing in a single nucleotide can be used to detect the presence of mutations in the target sequences because of the corresponding changes in the electrophoretic mobilities of the hetero- and homoduplexes. The fragments to be analyzed, usually PCR products, are xe2x80x9cclampedxe2x80x9d at one end by a long stretch of G-C base pairs (30-80) to allow complete denaturation of the sequence of interest without complete dissociation of the strands. The attachment of a GC xe2x80x9cclampxe2x80x9d to the DNA fragments increases the fraction of mutations that can be recognized by DGGE (Abrams et al., Genomics 7:463 [1990]). Attaching a GC clamp to one primer is critical to ensure that the amplified sequence has a low dissociation temperature (Sheffield et al., Proc. Natl. Acad. Sci., 86:232 [1989]; and Lerman and Silverstein, Meth. Enzymol. 155:482 [1987]). Modifications of the technique have been developed, using temperature gradient gels (Wartell et al., Nucl. Acids Res. 18:2699-2701 [1990]), and the method can be also applied to RNA:RNA duplexes (Smith et al., Genomics 3:217 [1988]).
Limitations on the utility of DGGE include the requirement that the denaturing conditions must be optimized for each specific nucleic acid sequence to be tested. Furthermore, the method requires specialized equipment to prepare the gels and maintain the high temperatures required during electrophoresis. The expense associated with the synthesis of the clamping tail on one oligonucleotide for each sequence to be tested is also a major consideration. In addition, long running times are required for DGGE. The long running time of DGGE was shortened in a modification of DGGE called constant denaturant gel electrophoresis (CDGE) (Borrensen et al., Proc. Natl. Acad. Sci. USA 88:8405 [1991]). CDGE requires that gets be performed under different denaturant conditions in order to reach high efficiency for the detection of unknown mutations. Both DGGE and CDGE are unsuitable for use in clinical laboratories.
An technique analogous to DGGE, termed temperature gradient gel electrophoresis (TGGE), uses a thermal gradient rather than a chemical denaturant gradient (Scholz, et al., Hum. Mol. Genet. 2:2155 [1993]). TGGE requires the use of specialized equipment which can generate a temperature gradient perpendicularly oriented relative to the electrical field. TGGE can detect mutations in relatively small fragments of DNA therefore scanning of large gene segments requires the use of multiple PCR products prior to running the gel.
Another common method, called xe2x80x9cSingle-Strand Conformation Polymorphismxe2x80x9d (SSCP) was developed by Hayashi, Selya and colleagues (reviewed by Hayashi, PCR Meth. Appl., 1:34-38, [1991]) and is based on the observation that single strands of nucleic acid can take on characteristic conformations under non-denaturing conditions, and these conformations influence electrophoretic mobility. The complementary strands assume sufficiently different structures that the two strands may be resolved from one another. Changes in the sequence of a given fragment will also change the conformation, consequently altering the mobility and allowing this to be used as an assay for sequence variations (Orita, et al., Genomics 5:874 [1989]).
The SSCP process involves denaturing a DNA segment (e.g., a PCR product) that is labelled on both strands, followed by slow electrophoretic separation on a non-denaturing polyacrylamide gel, so that intra-molecular interactions can form and not be disturbed during the run. This technique is extremely sensitive to variations in gel composition and temperature. A serious limitation of this method is the relative difficulty encountered in comparing data generated in different laboratories, under apparently similar conditions.
The dideoxy fingerprinting (ddF) technique is another technique developed to scan genes for the presence of unknown mutations (Liu and Sommer, PCR Methods Appli., 4:97 [1994]). The ddF technique combines components of Sanger dideoxy sequencing with SSCP. A dideoxy sequencing reaction is performed using one dideoxy terminator and then the reaction products are electrophoresised on nondenaturing polyacrylamide gels to detect alterations in mobility of the termination segments as in SSCP analysis. While ddF is an improvement over SSCP in terms of increased sensitivity, ddF requires the use of expensive dideoxynucleotides and this technique is still limited to the analysis of fragments of the size suitable for SSCP (i.e., fragments of 200-300 bases for optimal detection of mutations).
In addition to the above limitations, all of these methods are limited as to the size of the nucleic acid fragment that can be analyzed. For the direct sequencing approach, sequences of greater than 600 base pairs require cloning, with the consequent delays and expense of either deletion sub-cloning or primer walking, in order to cover the entire fragment. SSCP and DGGE have even more severe size limitations. Because of reduced sensitivity to sequence changes, these methods are not considered suitable for larger fragments. Although SSCP is reportedly able to detect 90% of single-base substitutions within a 200 base-pair fragment, the detection drops to less than 50% for 400 base pair fragments. Similarly, the sensitivity of DGGE decreases as the length of the fragment reaches 500 base-pairs. The ddF technique, as a combination of direct sequencing and SSCP, is also limited by the relatively small size of the DNA that can be screened.
Another method of detecting sequence polymorphisms based on the conformation assumed by strands of nucleic acid is the Cleavase(copyright) Fragment Length Polymorphism (CFLP(copyright)) method (Brow et al., J. Clin. Microbiol. 34:3129 [1996]; PCT International Application No. PCT/US95/14673 [WO 96/15267]; co-pending application Ser. Nos. 08/484,956 and 08/520,946). This method uses the actions of a structure specific nuclease to cleave the folded structures, thus creating a set of product fragments that can by resolved by size, e.g., by electrophoresis. This method is much less sensitive to size so that entire genes, rather than gene fragments, may be analyzed.
In many situations, e.g., in many clinical laboratories, electrophoretic separation and analysis may not be technically feasible, or may not be able to accommodate the processing of a large number of samples in a cost-effective manner. There is a clear need for a method of analyzing the characteristic conformations of nucleic acids without the need for either electrophoretic separation of conformations or fragments or for elaborate and expensive methods of visualizing gels (e.g., darkroom supplies, blotting equipment or fluorescence imagers).
The present invention relates to methods and compositions for treating nucleic acid, and in particular, methods and compositions for detection and characterization of nucleic acid sequences and sequence changes. The present invention provides a method for examining the conformations assumed by single strands of nucleic acid, forming the basis of a novel method of detection of specific nucleic acid sequences. The present invention contemplates use of the novel detection method for, among other uses, clinical diagnostic purposes, including but not limited to the detection and identification of pathogenic organisms.
The present invention contemplates using the interactions between probe oligonucleotides and folded nucleic acid strands in methods for detection and characterization of nucleic acid sequences and sequence changes. A complex formed by the specific interaction (i.e., reproducible and predictable under a given set of reaction conditions) of a probe that is at least partially complementary to a target nucleic acid sequence is referred to herein as a xe2x80x9cprobe/folded target nucleic acid complex.xe2x80x9d The interactions contemplated may be a combination of standard hybridization of oligonucleotides to contiguous, co-linear complementary bases, or may include standard basepairing to noncontiguous regions of complementarity on a strand of nucleic acid to be analyzed. In this context, the term xe2x80x9cstandard base pairingxe2x80x9d refers to hydrogen bonding that occurs between complementary bases, adenosine to thymidine and guanine to cytosine to form double helical structures of the A or B form. Such standard base pairing may also be referred to as Watson-Crick base pairing. It is contemplated that the interactions between the oligonucleotides of the present invention (i.e., the probes and the targets) may include non-standard nucleic acid interactions known in the art, such as triplex structures, quadraplex aggregates, and the multibase hydrogen bonding such as is observed within nucleic acid tertiary structures, such as those found in tRNAs.
In another embodiment, this mixture is present in an aqueous solution. The invention is not limited by the nature of the aqueous solution employed. The aqueous solution may contain mono- and divalent ions, non-ionic detergents, buffers, stabilizers, etc.
The present invention provides a method, comprising: a) providing: i) a folded target having a deoxyribonucleic acid (DNA) sequence comprising one or more double stranded regions and one or more single stranded regions; and ii) one or more oligonucleotide probes complementary to at least a portion of said folded target; and b) mixing said folded target and said one or more probes under conditions such that said probe hybridizes to said folded target to form a probe/folded target complex. The degree of complementarity between the probes and the target nucleic acids may be complete or partial (e.g., contain at least one mismatched base pair). The method is not limited by the nature of the target DNA employed to provide the folded target DNA. In one embodiment, the target DNA comprises single-stranded DNA. In another embodiment, the target DNA comprises double-stranded DNA. Folded target DNAs may be produced from either single-stranded or double-stranded target DNAs by denaturing (e.g., heating) the DNA and then permitting the DNA to form intra-strand secondary structures. The method is not limited by the manner in which the folded target DNA is generated. The target DNA may be denatured by a variety of methods known to the art including heating, exposure to alkali, etc. and then permitted to renature under conditions that favor the formation of intra-strand duplexes (e.g., cooling, diluting the DNA solution, neutralizing the pH, etc.).
The method is also not limited by the nature of the oligonucleotide probes; these probes may comprise DNA, RNA, PNA and combinations thereof as well as comprise modified nucleotides, universal bases, adducts, etc.
In a preferred embodiment, the method further comprises detecting the presence of said probe/folded target complex. When a detection step is employed either the probe or the target DNA (or both) may comprise a label (i.e., a detectable moiety); the invention is not limited by the nature of the label employed or the location of the label (i.e., 5xe2x80x2 end, 3xe2x80x2 end, internal to the DNA sequence) A wide variety of suitable labels are known to the art and include fluorescein, tetrachlorofluorescein, hexachlorofluorescein, Cy3, Cy5, digoxigenin, radioisotopes (e.g., 32P, 35S). In another preferred embodiment, the method further comprises quantitating the amount of probe/folded target complex formed. The method is not limited by the means used for quantitification; when a labeled folded target DNA is employed (e.g., fluorescein or 32P), the art knows means for quantification (e.g., determination of the amount of fluorescence or radioactivity present in the probe/folded target complex).
In a preferred embodiment, the probe in the probe/folded target complex is hybridized to a single stranded region of said folded target. In another preferred embodiment, the probe comprises an oligonucleotide having a moiety that permits its capture by a solid support. The invention is not limited by the nature of the moiety employed to permit capture. Numerous suitable moieties are known to the art, including but not limited to, biotin, avidin and streptavidin. Further, it is known in the art that many small compounds, such as fluorescein and digoxigenin may serve as haptens for specific capture by appropriate antibodies. Protein conjugates may also be used to allow specific capture by antibodies.
In a preferred embodiment the detection of the presence of said probe/folded target complex comprises exposing said probe/folded target complex to a solid support under conditions such that said probe is captured by said solid support. As discussed in further detail below, numerous suitable solid supports are known to the art (e.g., beads, particles, dipsticks, wafers, chips, membranes or flat surfaces composed of agarose, nylon, plastics such as polystyrenes, glass or silicon) and may be employed in the present methods.
In a particularly preferred embodiment, the moiety comprises a biotin moiety and said solid support comprises a surface having a compound capable of binding to said biotin moiety, said compound selected from the group consisting of avidin and streptavidin.
In another embodiment, the folded target comprises a deoxyribonucleic acid sequence having a moiety that permits its capture by a solid support; as discussed above a number of suitable moieties are known and may be employed in the present method. In yet another embodiment, the detection of the presence of said probe/folded target complex comprises exposing said probe/folded target complex to a solid support under conditions such that said folded target is captured by said solid support. In a preferred embodiment, the moiety comprises a biotin moiety and said solid support comprises a surface having a compound capable of binding to said biotin moiety, said compound selected from the group consisting of avidin and streptavidin.
In a preferred embodiment, the probe is attached to a solid support; the probe is attached to the solid support in such a manner that the probe is available for hybridization with the folded target nucleic acid the invention is not limited by the means employed to attach the probe to the solid support. The probe may be synthesized in situ on the solid support or the probe may be attached (post-synthesis) to the solid support via a moiety present on the probe (e.g., using a biotinylated probe and solid support comprising avidin or streptavidin). In another preferred embodiment, the folded target nucleic acid is attached to a solid support; this may be accomplished for example using moiety present on the folded target (e.g., using a biotinylated target nucleic acid and solid support comprising avidin or streptavidin).
The present invention also provides a method, comprising: a) providing: i) a first folded target having a nucleic acid sequence comprising first and second portions, said first and second portions each comprising one or more double stranded regions and one or more single stranded regions; ii) a second folded target having a nucleic acid sequence comprising a first portion that is identical to said first portion of said first folded target and a second portion that differs from said second portion of said first folded target because of a variation in nucleic acid sequence relative to said first folded target, said first and second portions each comprising one or more double stranded regions and one or more single stranded regions; iii) first and second oligonucleotide probes, said first oligonucleotide probe complementary to said first portion of said first and second folded targets and said second oligonucleotide probe complementary to said second portion of said first and second folded targets; and iv) a solid support comprising first, second, third and fourth testing zones, each zone capable of capturing and immobilizing said first and second oligonucleotide probes; b) contacting said first folded target with said first oligonucleotide probe under conditions such that said first probe binds to said first folded target to form a probe/folded target complex in a first mixture; c) contacting said first folded target with said second oligonucleotide probes under conditions such that said second probe binds to said first folded target to form a probe/folded target complex in a second mixture; d) contacting said second folded target with said first oligonucleotide probe to form a third mixture; e) contacting said second folded target with said second oligonucleotide probe to form fourth mixture; and f) adding said first, second, third and fourth mixtures to said first, second, third and fourth testing zones of said solid support, respectively, under conditions such that said probes are captured and immobilized. The degree of complementarity between the probes and the target nucleic acids may be complete or partial (e.g., contain at least one mismatched base pair).
In a preferred embodiment, the first probe in step d) does not substantially hybridize to said second folded target; that is while it is not required that absolutely no formation of a first probe/second folded target complex occurs, very little of this complex is formed. In another preferred embodiment, the hybridization of said first probe in step d) to said second folded target is reduced relative to the hybridization of said first probe in step c) to said first folded target.
The method is not limited by the nature of the first and second targets. The first and second targets may comprise double- or single-stranded DNA or RNA. The method is also not limited by the nature of the oligonucleotide probes; these probes may comprise DNA, RNA, PNA and combinations thereof as well as comprise modified nucleotides, universal bases, adducts, etc. In a preferred embodiment, the first and second oligonucleotide probes comprise DNA.
The present invention further provides a method, comprising: a) providing: i) a first folded target having a nucleic acid sequence comprising first and second portions, said first and second portions each comprising one or more double stranded regions and one or more single stranded regions; ii) a second folded target having a nucleic acid sequence comprising a first portion that is identical to said first portion of said first folded target and a second portion that differs from said second portion of said first folded target because of a variation in nucleic acid sequence relative to said first folded target, said first and second portions each comprising one or more double stranded regions and one or more single stranded regions; iii) a solid support comprising first and second testing zones, each of said zones comprising immobilized first and second oligonucleotide probes, said first oligonucleotide probe complementary to said first portion of said first and second folded targets and second oligonucleotide probe complementary to said second portion of said first and second folded targets; and b) contacting said first and second folded targets with said solid support under conditions such that said first and second probes hybridize to said first folded target to form a probe/folded target complex. The invention is not limited by the nature of the first and second folded targets. The first and second targets may be derived from double- or single-stranded DNA or RNA. The probes may be completely or partially complementary to the target nucleic acids. The method is also not limited by the nature of the oligonucleotide probes; these probes may comprise DNA, RNA, PNA and combinations thereof as well as comprise modified nucleotides, universal bases, adducts, etc. In a preferred embodiment, the first and second oligonucleotide probes comprise DNA. The invention is not limited by the nature of the solid support employed as discussed above.
In a preferred embodiment, the contacting of step b) comprises adding said first folded target to said first testing zone and adding said second folded target to said second testing zone. In another preferred embodiment, the first and second probes are immobilized in separate portions of said testing zones.
In a preferred embodiment, the first probe in said second testing zone does not substantially hybridize to said second folded target; that is while it is not required that absolutely no formation of a first probe/second folded target complex occurs, very little of this complex is formed. In another preferred embodiment, the first probe in said second testing zone hybridizes to said second folded target with a reduced efficiency compared to the hybridization of said first probe in first testing zone to said first folded target.
In one embodiment, the first and second folded targets comprise DNA. In another embodiment, the first and second folded targets comprise RNA.
The present invention also provides a method for treating nucleic acid, comprising: a) providing: i) a nucleic acid target and ii) one or more oligonucleotide probes; b) treating the nucleic acid target and the probes under conditions such that the target forms one or more folded structures and interacts with one or more probes; and c) analyzing the complexes formed between the probes and the target. In a preferred embodiment, the method further comprises providing a solid support for the capture of the target/probe complexes. Such capture may occur after the formation of the structures, or either the probe or the target my be bound to the support before complex formation.
The method is not limited by the nature of the nucleic acid target employed. In one embodiment, the nucleic acid of step (a) is substantially single-stranded. In another embodiment, the nucleic acid is RNA or DNA. It is contemplated that the nucleic acid target comprise a nucleotide analog, including but not limited to the group comprising 7-deaza-dATP, 7-deaza-dGTP and dUTP. The nucleic acid target may be double stranded. When double-stranded nucleic acid targets are employed, the treating of step (b) comprises: i) rendering the double-stranded nucleic acid substantially single-stranded; and ii) exposing the single-stranded nucleic acid to conditions such that the single-stranded nucleic acid has secondary structure. The invention is not limited by the method employed to render the double-stranded nucleic acid substantially single-stranded; a variety of means known to the art may be employed. A preferred means for rendering double stranded nucleic acid substantially single-stranded is by the use of increased temperature.
In a preferred embodiment, the method further comprises the step of detecting said one or more target/probe complexes. The invention is not limited by the methods used for the detection of the complex(es).
It is contemplated that the methods of the present invention be used for the detection and identification of microorganisms. It is contemplated that the microorganism(s) of the present invention be selected from a variety of microorganisms; it is not intended that the present invention be limited to any particular type of microorganismn. Rather, it is intended that the present invention will be used with organisms including, but not limited to, bacteria, fungi, protozoa, ciliates, and viruses. It is not intended that the microorganisms be limited to a particular genus, species, strain, or serotype. Indeed, it is contemplated that the bacteria be selected from the group comprising, but not limited to members of the genera Campylobacter, Escherichia, Mycobacterium, Salmonella Shigella, and Staphylococcus. In one preferred embodiment, the microorganism(s) comprise strains of multidrug resistant Mycobacterium tuberculosis. It is also contemplated that the present invention be used with viruses, including but not limited to hepatitis C virus, human immunodeficiency virus and simian immunodeficiency virus.
Another embodiment of the present invention contemplates a method for detecting and identifying strains of microorganisms, comprising the steps of extracting nucleic acid from a sample suspected of containing one or more microorganisms; and contacting the extracted nucleic acid with one or more oligonucleotide probes under conditions such that the extracted nucleic acid forms one or more secondary structures and interacts with one or more probes. In one embodiment, the method further comprises the step of capturing the complexes to a solid support. In yet another embodiment, the method further comprises the step of detecting the captured complexes. In one preferred embodiment, the present invention further comprises comparing the detected from the extracted nucleic acid isolated from the sample with separated complexes derived from one or more reference microorganisms. In such a case the sequence of the nucleic acids from one or more reference microorganisms may be related but different (e.g., a wild type control for a mutant sequence or a known or previously characterized mutant sequence).
In an alternative preferred embodiment, the present invention further comprises the step of isolating a polymorphic locus from the extracted nucleic acid after the extraction step, so as to generate a nucleic acid target, wherein the target is contacted with one or more probe oligonucleotides. In one embodiment, the isolation of a polymorphic locus is accomplished by polymerase chain reaction amplification. In an alternate embodiment, the polymerase chain reaction is conducted in the presence of a nucleotide analog, including but not limited to the group comprising 7-deaza-dATP, 7-deaza-dGTP and dUTP. It is contemplated that the polymerase chain reaction amplification will employ oligonucleotide primers matching or complementary to consensus gene sequences derived from the polymorphic locus. In one embodiment, the polymorphic locus comprises a ribosomal RNA gene. In a particularly preferred embodiment, the ribosomal RNA gene is a 16S ribosomal RNA gene.
The present invention also contemplates a process for creating a record reference library of genetic fingerprints characteristic (i.e., diagnostic) of one or more alleles of the various microorganisms, comprising the steps of providing a nucleic acid target derived from microbial gene sequences; comprising the steps of extracting nucleic acid from a sample suspected of containing one or more microorganisms; and contacting the extracted nucleic acid with one or more oligonucleotide probes under conditions such that the extracted nucleic acid forms one or more secondary structures and interacts with one or more probes; detecting the captured complexes; and maintaining a testable record reference of the captured complexes.
By the term xe2x80x9cgenetic fingerprintxe2x80x9d it is meant that changes in the sequence of the nucleic acid (e.g., a deletion, insertion or a single point substitution) alter both the sequences detectable by standard base pairing, and alter the structures formed, thus changing the profile of interactions between the target and the probe oligonucleotides (e.g., altering the identity of the probes with which interaction occurs and/or altering the site/s or strength of the interaction) The measure of the identity of the probes bound and the strength of the interactions constitutes an informative profile that can serve as a xe2x80x9cfingerprintxe2x80x9d of the nucleic acid, reflecting the sequence and allowing rapid detection and identification of variants.
The methods of the present invention allow for simultaneous analysis of both strands (e.g., the sense and antisense strands) and are ideal for high-level multiplexing. The products produced are amenable to qualitative, quantitative and positional analysis. The present methods may be automated and may be practiced in solution or in the solid phase (e.g., on a solid support). The present methods are powerful in that they allow for analysis of longer fragments of nucleic acid than current methodologies.